1. Field of the Invention
The present invention relates to image forming apparatus adapted for the reversal developing process.
2. Description of the Prior Art
Conventional image forming apparatus, such as laser beam printers, which are adapted to practice the reversal developing process comprise a photosensitive member rotatably provided centrally thereof, and a sensitizing charger, developing roller, transfer charger, cleaner and eraser lamp which are successively arranged around the photosensitive member along the direction of rotation thereof. These components of the conventional image forming apparatus are controlled with the timing illustrated in FIG. 1. When the main motor is turned on first, the photosensitive member starts rotation, and the developing roller, sensitizing charger and eraser lamp are turned on at the same time. Subsequently, the transfer charger is turned on with predetermined timing. The rear end of an electrostatic latent image thereafter passes through the transfer position, whereupon the transfer charger is turned off. When a sheet is completely discharged from the apparatus next, the main motor is stopped, and the developing roller, eraser lamp and sensitizing charger are turned off at the same time.
With the image forming apparatus described, corona discharge of the sensitizing and transfer chargers produces ozone (O.sub.3) and nitrogen oxides (NO.sub.x), which will act on talc, kaoline or like component contained in paper particles deposited from the sheet onto the surface of the photosensitive member to reduce the sensitivity of the photosensitive member at the portion thereof where the paper particles are deposited. Consequently, the surface potential of the particle bearing portion will not fully drop when exposed to a laser beam.
In the case where the potential thus fails to drop sufficiently owing to a reduction in the sensitivity of the photosensitive member, the following problem arises. When the surface of the photosensitive member as uniformly charged to a predetermined potential V0 is irradiated with the laser beam, the surface potential of the irradiated portion usually decreases to a potential Vi lower than the developing bias VB as indicated in a solid line in FIG. 2 to form an electrostatic latent image. Toner adheres to the portion lower than the developing bias VB in potential to form a toner image. However, if the portion of reduced sensitivity is irradiated with the laser beam, the surface potential of this portion decreases only to a potential Vj which is higher than the developing bias VB as indicated in a broken line in FIG. 2, with the result that no toner adheres to the portion to create a fault in the image. Such a fault occurs regardless of whether a plurality of images are produced continually or intermittently, whereas the fault becomes more pronounced if the length of time after the completion of one image until the formation of the next image is shorter.
This problem will be overcome by turning off the sources of ozone and nitrogen oxides, i.e., the sensitizing charger and the transfer charger, immediately after the rear end of the electrostatic latent image has moved past the transfer charger. On the other hand, the surface of the photosensitive member moving past the transfer charger before the transfer charger is turned off is subjected by this charger to corona discharge opposite in polarity to the corona discharge of the sensitizing charger and has its potential reduced to a level lower than the developing bias VB. Further with usual image forming apparatus, the main motor continues rotation to rotate the photosensitive member a plurality turns until the sheet is completely discharged from the apparatus after the latent image rear end has moved past the transfer charger. Accordingly, in the case where the sensitizing charger and the transfer charger are simultaneously turned off immediately after the latent image rear end has moved past the transfer charger, the portion of the photosensitive member, having its potential reduced by moving past the transfer charger immediately before the transfer charger is turned off and retaining the reduced potential, passes by the developing roller of the reversal developing unit. The toner then adheres to this portion and becomes wasted uselessly.